1. Field of the Invention
This invention relates to an exposure time control device which is arranged to correct an exposure error resulting from overlapping of the curtains of a focal plane shutter, and the like.
2. Description of the Prior Art
Generally, exposure time control for a camera having a focal plane shutter is performed as follows: An exposure time controlling timer circuit is actuated to perform a time counting operation while a leading shutter curtain is allowed to travel by a shutter release of the camera; upon termination of an exposure time counted by the timer circuit, say, 1/1000 sec., 1/500 sec., etc., a trailing shutter curtain is allowed to travel. However, in a focal plane shutter, the leading and trailing curtains of the shutter must be partially overlapped at the time of shutter charging and before commencement of a shutter operation. Besides, there is some response delay of the magnet arranged to hold the trailing curtain. Accordingly, the starting positions of the leading and trailing shutter curtains differ from each other, while the response delay of the magnet makes it impossible to have the actual length of film exposure time coincide with the counted length of exposure time. Therefore, it has been impossible to accomplish precise exposure control through the mere arrangement of starting a time counting operation concurrently with the travel of the leading shutter curtain, and allowing the trailing shutter curtain to travel upon completion of the time counting operation.
To solve this problem, in the conventional cameras using focal plane shutters, an adjusting time Tx is arranged in addition to the exposure time. More specifically, the delay time Tmg of the magnet, the overlapping time Tg of the shutter curtains, the exposure count time Te and the actual length of film exposing time TA are in the following relation: TA+Tg =Te+Tmg. The film exposing time TA, therefore, can be expressed as follows: TA=Te+Tmg-Tg. It is impossible to obtain a relation of TA=Te. The exposure count time and the film exposing time thus never coincide with each other. Accordingly, it is necessary to obtain the relation of TA=Te by adjusting the travelling time of the shutter curtains and the counting time. However, to accomplish this adjustment in a mechanical manner is difficult because the overlapping degree of shutter curtains and the response delay of the magnet of one camera differ from those of another. To overcome this difficulty, in the conventionally practiced method, an adjusting time Tx is added to the count time Te through a timer circuit; and the relation of TA=Te is obtained by actuating the trailing curtain holding magnet after the lapse of a length of time Te+Tx (hereinafter this method will be called the plus exposure time adjusting method).
In other words, the conventional devices are arranged to have a relation of Te+Tx+Tmg=Tg+TA and the Tx is adjusted to become Tg-Tmg through the timer circuit. A relation of Te=TA is realized by equalizing the count time Te with the film exposing time TA through the above stated adjustment.
The adverse effect which is caused to take place to uneven degrees in cameras, by the overlapping time Tg of the shutter curtains and the response delay time Tmg of the magnet, thus has been simply coped with by equalizing the count time and the film exposing time to each other through the adjustment time Tx=Tg-Tmg provided by the timer conduit.
However, even with the above stated method, the relation of Te=TA becomes hardly realizable, and there arises an exposure error in cases where the shutter curtain overlapping time Tg is shorter than the response delay time Tmg of the magnet. In other words, since the adjustment time Tx is Tg-Tmg, in the case of Tg&lt;Tmg, the adjustment time Tx becomes Tx&lt;0 which makes the adjustment hardly possible. To solve this problem, an exposure control circuit has recently been proposed as disclosed in a U.S. patent application, Ser. No. 144,981, filed Apr. 30, 1980. In accordance with the method of this exposure control circuit (hereinafter called the minus exposure time adjusting method), the count time Te for the exposure is arranged to be shorter than the film exposing time TA for exposing the film surface to a proper degree, such as 1/1000, 1/500, etc., by a predetermined length of time Tc, such as 0.5 msec. In short, the above stated problem is solved by obtaining a relation of TA-Tc=Te.
More specifically stated, in the formula given above, Te+Tmg+Tx=Tg+TA, Te is arranged to be TA-Tc to obtain a relation of TA-Tc+Tmg+Tc. With this arrangement employed, even when an increase in the shutter curtain travelling speed results in a relation of Tg&lt;Tmg, time adjustment can be accomplished through the adjustment time Tx so long as the relation does not become Tg+Tc&lt;Tmg. Therefore, in accordance with the minus exposure time adjusting method, the exposure error due to the adjustment time Tx can be corrected. Even when an increase in the travelling speed of the shutter curtains causes the delay time of the magnet to become longer than the overlapping time of the shutter curtains, the exposure error due to the overlapping of the shutter curtains and the response delay of the magnet can be corrected by the adjustment time Tx.
However, since the exposure count time Te is arranged to be shorter than the exposure time TA by a certain predetermined length of time Te in accordance with the proposed method, there is a certain limit to the predetermined time Tc. For example, the limit to the time Tc is 1 msec where the highest shutter speed or time is 1/1000 sec and is 0.5 msec where the highest shutter speed is 1/2000. The time Tc is thus limited by the highest shutter operating speed. Accordingly, adjustment becomes impossible and an exposure error inevitably arises if this limit is exceeded.